You put a container around the coral so that during feeding you can concentrate the food around the polyps for them to feed. othewise, the food gets all blown in with tank water, and is a lower concentration, and this harder for the corals to feed.
The problem with a tank of non-photosyntheitc corals would be water quality. You need enough food in the tank for these corals to survive and grow, but at the same time, you need to keep your water parameters up...which goes against adding tons of food to a tank.
I would do a fair amoount of research before buying these animals, as they all have specific feeding requirements. Sun corals as mentioned, are not hard, but they do take time to feed on a regular basis.
Other corals, like some gorgonians are far harder to care for, and require the corret partical size food, at the correct flow rate, etc. These are not easy corals to keep.
HTH
Stewart